It looks like you are forgetting to select PP"X", you need to select the "next" PP1/PP2 after each drop. Every station can have more than one tgt loaded as you are aware off. But you need to switch between both, the PP tgts manually.
PP1 then drop
step to next station
PP1 then drop
PP2 drop
it will automatically switch to the last station so all you need to do for the last bomb is select PP2 and voila.